This invention relates to a vehicle brake device having a hydraulic pressure generating source for generating brake hydraulic pressure, particularly to a vehicle brake device which can prevent dragging of brake pads.
Vehicle brake devices are known in which a brake fluid circulating passage including a hydraulic pressure generating source is provided in a brake fluid supply passage for supplying brake fluid from a master cylinder to wheel cylinders to assist brake operations of the driver, and the operating force or operating amount of the brake pedal is detected as electric signals to drive the hydraulic pressure generating source based on these electric signals, thereby carrying out control of the brake hydraulic pressure while sensing the intention of the driver.
Further, there are also known vehicle brake devices having an automatic pressure increase function in which a communication valve is provided between the discharge side of the hydraulic pressure generating source and the master cylinder, and a shutoff valve is provided in a passage through which the intake side of the hydraulic pressure generating source and the master cylinder communicates to drive the hydraulic pressure generating source according to the travel state of the vehicle, which is detected as electric signals, thereby controlling the brake hydraulic pressure.
On the other hand, in vehicle brake devices in which brake pads are pressed against each disk rotor under the brake hydraulic pressure of a wheel cylinder, brake pads may contact the disk rotor even though the brake pedal is not operated due e.g. to uneven wear of the brake pads, thus causing so-called dragging. Such dragging causes lowering of fuel cost and further quicken uneven wear of the brake pads.
In order to prevent such dragging, there is known one in which using the above vehicle brake device which has a hydraulic pressure generating source, a pump as the hydraulic pressure generating source is driven for a predetermined time while the brake pedal is being released and the accelerator pedal is being operated to reduce the brake pressure of the wheel cylinders, thereby increasing the distances between the brake pads and the disk rotor (e.g. see JP patent publication 2002-347598 (pages 3-7, FIGS. 4-5).
In such a conventional dragging preventive means which uses a hydraulic pressure generating source, every time the accelerator pedal is operated during travel of the vehicle, the pump as the hydraulic pressure source is frequently activated. Thus, in order to ensure durability of the pump, it is necessary to employ a brushless motor or a motor having a large brush. This increases the price and size of the motor. Further, operating sounds of the pump will give uncomfortableness to the driver.
Further, if the distances between the brake pads and the disk rotors increase while the accelerator pedal is being operated, initial response of the brakes during braking tends to be delayed. This prolongs the braking distance. Further, the initial useless stepping feeling when the brake pedal is stepped in is large. Thus light brake operating feeling may be impaired.
An object of this invention is to properly prevent dragging without a waste and ensure initial response of brakes during braking by using a vehicle brake device having a hydraulic pressure generating source.